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The present invention relates to suspension systems for off-road equipment, such as agricultural tractors, and more particularly to such suspension systems that provide hydraulic load leveling.
Off-road equipment, such as construction and agricultural vehicles, can carry widely varying loads. When a relatively heavy load is applied to the equipment, the vehicle body is forced downward with respect to the axles supporting the wheels on which the vehicle rides. This results in compression of the suspension which can adversely affect the maneuverability of the vehicle. On the other hand, if the suspension is configured for very heavy loads, the vehicle may have an undesirable ride under light load conditions.
As a result, many vehicles have automatic load leveling systems which employ one or more hydraulic cylinders between the axle and the frame of the vehicle to ensure that the frame is maintained at the proper height above the axle. When a heavy load is applied to the frame, the drop of the frame is sensed and additional hydraulic fluid is applied to the cylinder to raise the frame the desired distance from the axle. Thereafter, when the load is removed from the vehicle the frame will rise significantly above the axle. When this occurs hydraulic fluid is applied to the opposing chamber of the cylinder to lower the frame with respect to the axle. This type of automatic hydraulic load leveling system ensures that the frame and axle will be at the desired separation regardless of the size of the load applied to the vehicle.
One of the drawbacks of this load leveling system is that the opposite chambers of the double acting cylinder have separate pressure control circuits and require high pump pressure to move the cylinder in both directions. Thus the consumption of fluid from the pump for load leveling may adversely affect the availability of fluid pressure for other functions powered by the tractor. In order to compensate for that power consumption, the pump capacity would have to be increased thus raising the cost of the hydraulic system.
Although the piston within the load leveling hydraulic cylinders moves under heavy loads, the piston does not move in response to the relatively small forces due to driving the vehicle over rough terrain. Therefore, the cylinders provide a very stiff the suspension system with negligible shock absorption. This results in a very rough ride, which can be uncomfortably for the operator.
A section of a hydraulic circuit is formed by a relief valve, an orifice and a check valve connected in parallel between two circuit nodes. Fluid flows through the check valve only from one circuit node to the other circuit node. The relief valve opens when the pressure at other circuit node is greater than a predefined level.
The components of this hydraulic circuit section are integrated into an assembly that is adapted to be placed in a bore of a housing that connects the two nodes. That valve assembly includes a body with a first end, a second end, and a intermediate section with a surface for engaging the housing when the body is located in the bore. At least one aperture extends through the intermediate section and a passage extending there through. A check valve member is attached to the body and closes the aperture when pressure in the bore adjacent to the second end is greater than pressure in the bore adjacent the first end.
A relief valve assembly is moveably attached to the body to form an opening at one end of the passage. The opening has a first cross sectional area when the relief valve assembly is in a first position which occurs when pressure in the bore adjacent to the second end is greater than a bias force. Under other pressure conditions, the relief valve assembly assumes a second position at which the opening has a second cross sectional area that is smaller than the first cross sectional area. This second cross sectional area defines the orifice of the hydraulic circuit section.